Effect of Scanning Strategy in the L-PBF Process of 18Ni300 Maraging Steel

Maraging steels are good candidates for the laser powder bed fusion process (L-PBF), also known as Selective Laser Melting, due to excellent weldability and resistance to quench cracking. Powders physical and chemical characteristics dominate the final microstructure and properties of the printed parts, that are also heavily influenced by the process parameters. In this study, the effects of the scanning strategies on dimensions, average surface roughness, density and material hardness were evaluated, keeping the powder type and the volumetric energy density (Andrew number) constant. The effects of the scanning strategy on these properties are far less understood than on other important ones, like residual stresses and distortion, strongly affected by the scanning strategy. In this study, parallel stripes, chessboard and hexagonal pattern strategies were studied, keeping the Andrew number constant but varying the interlayer rotation. In general, the hexagonal strategy underperformed compared to the chessboard and the stripes ones.

Quench Process and Steel Chemistry Optimization to Prevent Quench Cracking during Hardening of Splined Semi – Axles

In the paper the hardening process of splined semi-axles is discussed and physics of preventing the quench crack formation during intensive quenching (IQ) is explained. It is shown that during IQ process at the splined cylindrical surface very high compressive current band residual stresses are formed which prevent the possibility of quench crack formation on splines. It is enough to optimize the stress distribution through the section of semi-axle and perform IQ process in order to prevent quench cracks formation in splines. It is achieved via optimizing depth of surface hardened layer. In this case the depth of surface hardened layer for cylinders and for cylinders with splines are the same. There is no need to create a special thin shell on splines or perform carburization to create such shell. Due to larger martensite specific volume, it results in surface compressive residual stress formation. Absence of martensite phase at the core eliminates core swelling that could be a reason in tensile surface stresses. The idea is supported by FEM calculations and testing of real semi-axles in industrial condition. The new idea simplifies cardinally technological process and makes it less costly.

An Approach to Micro Segregation Level and Presence of Quench Cracks in Medium Carbon Low Alloy Steels

Samples from forged and heat-treated steel products with known quench crack histories have been mapped in order to study a possible relation between banding segregation and quench cracking. The steels were medium carbon low alloy steels, ingot and continuous cast, as well as vacuum arc remelt D6AC. EDS X-ray mapping was applied to characterize the banding segregation pattern from casting i.e. gradient of chemical composition that creates direction dependent properties. Trends for segregation ratios followed the expectations: the segregation ratios were higher for the quench sensitive steels.34CrNiMo6, ingot- and continuous cast, was then supplied for testing. Segregation level was pre-checked for decision for heat treatment and testing of as quenched tensile properties. There is an indication of 90 % reduction of the difference between Rm and Rp0.2, the work hardening, for the steel with lowest quality.

Rolled Metal Structure Resource Saving Preparation for Long-Length Hardened Bolt Upsetting

Along with design factors, reliability of the work-pieces for machines and mechanisms is a function of the structure and the properties of the implied materials; these properties depend significantly on the kind and the mode of the process treatment. Taking into account hardwares production, along with the increased structural integrity and service reliability, a reduction of costs starts from the rolled metal production to manufacturing of the finished parts of required quality. This is particularly relevant under the conditions of mass-production. The article presents a resource saving process flow for a 40X rolled steel, developed as a result of studying the influence of thermal and plastic treatment modes on its structural and mechanical properties. The purpose of the new process flow is the production of hardened bolts, involving drawing and isothermal treatment, but excluding the volume quenching and tempering. Spheroidizing annealing is substituted with an isothermal operation, patenting, which allows to reduce energy costs and to increase environmental compatibility of production and service reliability of bolted items. This makes it possible to shorten the process chain and to reduce the cost of long-length bolts production. The method prevents any potential quench cracking, eliminates the necessity for work-pieces' straightening, decreases the risk of thread defects.